User Terminal, Radio Base Station Apparatus, Radio Communication System and Radio Communication Method

1. A radio communication system comprising: a radio base station apparatus comprising: an allocation section that allocates downlink control information to a first control region from a top to a predetermined OFDM symbol in a subframe, and also allocates, in a specific subframe, downlink control information to a second control region that is frequency-divided with a data region; and a transmission section that transmits information indicating whether a user terminal needs to decode downlink control information in the specific subframe, the downlink control information allocated in the first control region and the downlink control information allocated in the second control region; and the user terminal comprising: a receiving section that receives the information indicating the specific subframe, the downlink control information of the first control region, and the downlink control information of the second control region from the radio base station apparatus; and a decoding section that decodes the downlink control information of the first control region and decodes the downlink control information of the second control region based on the information indicating the specific subframe, wherein, for allocation of the downlink control information to the second control region, there are defined in advance a first method of allocating a plurality of resource element groups constituting a control channel element used for the downlink control information of the second control region to over a plurality of physical resource blocks (PRBs) and a second method of allocating the downlink control information of the second control region in PRB units, and the allocation section performs the allocation by selecting either the first method or the second method, and wherein the transmission section provides the information indicating the specific subframe in which the second control region is configured to the user terminal by higher layer signals.

2. The radio communication system according to claim 1 , wherein the transmission section further transmits information about resource blocks to configure the second control region in the specific subframe by higher layer signaling.

3. A radio base station apparatus comprising: an allocation section that allocates downlink control information to a first control region from a top to a predetermined OFDM symbol in a subframe, and also allocates, in a specific subframe, downlink control information to a second control region that is frequency-divided with a data region; and a transmission section that transmits information indicating whether a user terminal needs to decode downlink control information in the specific subframe, the downlink control information allocated in the first control region and the downlink control information allocated in the second control region to the user terminal, wherein, for allocation of the downlink control information to the second control region, there are defined in advance a first method of allocating a plurality of resource element groups constituting a control channel element used for the downlink control information of the second control region to over a plurality of physical resource blocks (PRBs) and a second method of allocating the downlink control information of the second control region in PRB units, and the allocation section performs the allocation by selecting either the first method or the second method, and wherein the transmission section provides the information indicating the specific subframe in which the second control region is configured to the user terminal by higher layer signals.

4. A user terminal comprising: a receiving section that receives downlink control information that is allocated to a first control region from a top to a predetermined OFDM symbol in a subframe, downlink control information that is allocated to a second control region that is frequency-divided with a data region in a specific radio frame, and information indicating the specific subframe; and a decoding section that decodes the downlink control information of the first control region and decodes the downlink control information of the second control region based on the information indicating the specific subframe, wherein the receiving section receives the information indicating whether the user terminal needs to decode downlink control information in the specific subframe in which the second control region is configured, by higher layer signals, and wherein, for allocation of the downlink control information to the second control region, there are defined in advance a first method of allocating a plurality of resource element groups constituting a control channel element used for the downlink control information of the second control region to over a plurality of physical resource blocks (PRBs) and a second method of allocating the downlink control information of the second control region in PRB units, and the allocation is performed by selecting either the first method or the second method.

5. A radio communication method between a radio base station apparatus and a user terminal, comprising the steps of: in the radio base station apparatus, allocating downlink control information to a first control region from a top to a predetermined OFDM symbol in a subframe and also allocating, in a specific subframe, downlink control information to a second control region that is frequency-divided with a data region; and transmitting information indicating whether the user terminal needs to decode downlink control information in the specific subframe, the downlink control information allocated in the first control region and the downlink control information allocated in the second control region; and in the user terminal, receiving the information indicating the specific subframe, the downlink control information of the first control region and the downlink control information of the second control region from the radio base station apparatus; and decoding the downlink control information of the first control region and decoding the downlink control information of the second control region based on the information indicating the specific subframe, wherein, for allocation of the downlink control information to the second control region, there are defined in advance a first method of allocating a plurality of resource element groups constituting a control channel element used for the downlink control information of the second control region to over a plurality of physical resource blocks (PRBs) and a second method of allocating the downlink control information of the second control region in PRB units, and the radio base station apparatus performs the allocation by selecting either the first method or the second method, and wherein the radio base station apparatus provides the information indicating the specific subframe in which the second control region is configured to the user terminal by higher layer signals.